oracle外键的约束?

create table a (shuzi number(4) primary key,gesu number(4));insert into taoge values(11,12）； insert into taoge values(22,23）； insert into taoge values(33,34）； insert into taoge values(44,45）； create table b( shuzi number(4) );insert into b values( 888);为什么写下alter table b addconstraint yuesu foreign key(shuzi)references a;语句时上面提示ORA-02298: 无法验证 (SYSTEM.YUESU) - 未找到父项关键字 收起